Electrically insulating coating



Patented a. 15, 1940 UNITED STATES PATENT OFFICE No Drawing. Application October 7, 1937, Serial No. 167,863. In Sweden October 28, 1936 Claims.

In electrical engineering it is often desirable to produce an electically insulating coating on metallic surfaces which will stand heating without undergoing any changes. This applies for instance to induction furnaces of different types, especially high-frequency furnaces, in which the primary coil, made, e. g., of copper tubes, is in the immediate vicinity of the melt in the furnace chamber.

Insulating media hitherto used, such as mica in combination with organic binders (shellac, compound, etc.) asbestos or doughy masses prepared with, e. g., water-glass as a binding medium, lose to a large extent their insulating power on heating, and do not therefore offer a suflicient guarantee against electric conduction. This is especially true, if the overheating is local, e. g., when the melt breaks through the furnace wall or the like.

Enamels are not suitable, because they must be heated to 300 or 400 C. before application, in which case, however, the copper or the like is damaged. Further, they are not fire-proof, but

begin to sinter, when the temperature stated is exceeded.

The present invention has been based on extensive reseaches and has for its object to eliminate the above-mentioned drawbacks, and con-- sists substantially in that the metallic surface is coated with a mass in the form of a paste or dough which, besides fillers, such as quartz or porcelain, contains, on the one hand, components rich in hydrates of aluminum oxide such as- J tradistinction to other methods a ceramic coating is thus produced on the metal in a simple manner by burning at a low temperature; the method'may therefore be used advantageously for coating, e. g., copper. Aluminum phosphate being in itself fire-proof and electrically insulating, it is possible by a suitable choice of the other components of the mass to produce a coating which will keep its insulating properties also on heating to a high temperature.

In adhering to the basic principle of the invention a great number of tests have been made for the purpose ofobtaining the most favorable results in the application of the method. It is desired, inter alia, that the mass (paste or dough) should have such a consistency that it 5 may be applied without difficulty on the metallic surface, that the massshould dry without the active constituents difiusing to the surface (effiorescing), that the mass should be set or fixed at the lowest possible temperature, and that the mass, after fixing, should be free from cracks and display great compactness and good adhesion to to the metal.

.The best results have been obtained with a combination of hydrates of aluminum oxide, such as bauxite or laterite, free kaolin and free phosphoric acid with, for instance, quartz as a filler. In this case the phosphoric acid, in a suitable dilution, is not added to the otherwise ready, dry, powdered mixture until immediately before use. The hydrate of aluminum oxide and the kaolin should be finely ground and enter the powdered mixture-in amounts of. to or 5 to 15 per cent. by weight, respectively. The grain size of the filler should be suited to the purpose in view. For a paste having a consistencypermitting application with a brush, quartz with a maximum grain size of 0.l5\ millimeter has proved suitable. For more viscous pastes, intended specially for use as adhesives between the turns of a coil, an additionof somewhat coarser grain sizes, e. g., to per cent by weight of a" grain size of 0.2 to 0.5 millimeter has given good results. The phosphoric acid is preferably added in the form of a solution prepared from or corresponding to 1 part by volume of concentrated phosphoric acid (specific gravity 1.70) diluted with 2 to 5 parts by volume of water.

The hydrate of aluminum oxide reacts on heating with the phosphoric acid with the formation of aluminum phosphate. The kaolin is used according to the invention for making the mass plastic and for counteracting the diffusion of the phosphoric acid to the surface during drying. The filler which consists of quartz, for, example, counteracts the formation of cracks and contributes to the compactness of the mass.

The tests have shown that when using'laterite, i. e., bauxite with a high content of boundwater (25 to 35 per cent), the fixation of the mass can be eifected at a temperature as low as 100 degrees centigrade, which has, among others, the advantage that it is possible in certain cases to use steam or water for the heating or the control of the temperature. The coating of a tube spiral may thus be fixed by steam led through the tube.

Excellent results have been obtained in insulating copper tubes with the following masses which set at about 100:

Example 1 A dough consisting of 20 parts by weight of bauxite with about 30 per cent. of bound water, 10 parts of kaolin and '70 parts of pure quartz crushed to a grain size less than 0.15 millimeter and, to each 100 grams of powdered mixture, about 30 cubic centimeters of phosphoric acid solution prepared from or corresponding to 1 part by volume of concentrated phosphoric acid (specific gravity 1.70) and 5 parts of water.

Example 2 A paste consisting of 20 parts byweight of bauxite with 30 per cent. of bound water, 5 parts of kaolin, 35 parts of quartz of a grain size less than 0.15 millimeter and 40 parts of quartz of a grain size of from 0.2 to 0.5 millimeter and, to each 100 grams of powdered mixturaabout 15 cubic centimeters of phosphoric acid solu--,

tion prepared from or corresponding to 1 part by volume of concentrated phosphoric acid (specific gravity 1.70) and 4 parts of water.

Crack-free coatings fixed at 100 degrees centigrade have been produced with these masses on copper tube coils, and have retained their electrio insulating power also on intense heating and at high electrical tensions between the coil turns. When the copper tube was cooled by circulating water, the surface of the coating could be heated up to melting-point (about 1600 degrees centigrade) without any electric leakage current occurring between the turns.

The above-mentioned applications of .the method must evidently be regarded as examples only. Good results may be obtained also with materials of other qualities and proportions.

What is claimed is:

1. In the production of electrically insulating coatings on metal surfac'es at low temperatures, the process which comprises coating a metal surface with a plastic composition containing a hydrate of aluminum oxide in amount ranging from about 15 to 30 per cent by weight, a refractory clay which is electrically insulating when in the dry state in amount ranging from about 5 to 15 per cent by weight, and a phosphoric acid-containing material capable of reacting with said aluminum oxide during the subsequent heating step with the formation of aluminum phosphate; the balance of said composition consisting sub-- stantially of an inert, refractory, granular filler; then drying and heating the coating at temperatures not substantially exceeding 300 C.

2. In the production of electrically insulating coatings on the copper coils of induction furnaces, the process which comprises coating the surface of such a coil with a composition com prising from about 15 to 30 partsby weight of a hydrate of aluminum oxide, about 5 to 15 parts by weight of a refractory clay which is electrical- 1y insulating in the dry state, about to 55 parts by weight of an inert, refractory, granular filler and phosphoric acid in a quantity sufiicient, upon drying and heating, to form aluminum phosphate from said hydrate of aluminum oxide, then drying and heating the coating at temperatures not substantially exceeding 300 C.

3. A plastic composition capable of setting to form a refractory and electrically insulating coating on metallic surfaces when heated to temperatures ranging from about to 300 C., comprising a mixture of a hydrate of aluminum oxide in amount ranging from about 15 to 30 per cent by weight, a refractory clay which is electrically insulating when in the dry state in amount ranging from about 5 to 15 per cent, an inert, refractory, granular filler forming substantially the balance of the composition, and phosphoric acid, said phosphoric acid being present in amount sufiicient to form aluminum phosphate from said hydrate of aluminum oxide when dried and heated.

4. A plastic composition capable of setting to form a refractory and electrically insulating coating on metallic surfaces when heated to temperatures ranging from about 100 to 300 C., comprising a mixture containing about 15 to 30 parts by weight of a hydrate of aluminum oxide, about 5 to 15 parts by weight of a refractory clay which is electrically insulating when in the dry state, about 80 to 55 parts by weight of an inert refractory, granular filler and phosphoric acid in a quantity sufficient, upon drying and heating,

to formaluminum phosphate from said hydrate of aluminum oxide.

5. The composition of claim 4 wherein said hydrate of aluminum oxide is bauxite.

6. The composition of claim 4 wherein said hydrate of aluminum oxide is laterite.

7. The composition of claim 4 wherein the grain size of said inert filler ranges up to 0.15 mm.

8. The composition of claim 4 wherein said filler is composed of two fractions having a different grain size, one fraction having a grain size ranging up to 0.15 mm. and the other fraction, constituting from about 40 to 50 per cent by weight of the filler, having a grain size of about 0.2 to 0.5 mm.

9. A plastic composition capableof setting to form a refractory and electrically insulating coating on metallic surfaces when heated to temperatures ranging from about 100 to 300 C., comprising about 20 parts by weight of bauxite, about 10 parts by weight of kaolin, about 70 parts by Weight of an inert, refractory, granular material having a grain size up to 0.15 mm. and a phosphoric acid solution containin about 1 part by volume of concentrated acid to 5 parts of water, in amount substantially sufficient to react with the bauxite to form aluminum phosphate.

10, A plastic composition capable of setting to form a refractory and electrically insulating coating on metallic surfaces when heated to temperatures ranging from about 100 to 300 C., comprising about 20 parts by weight of bauxite, 5 parts by weight of kaolin, about 35 parts by weight of an inert, refractory, granular material having a grain size up to 0.15 mm. and a phosphoric acid solution, containing'about 1 part of concentrated acid to 4 parts of water, in amount substantially sufiicient to react with the bauxite to form aluminum phosphate.

BERTIL STALHAN'E. 

